Wind turbines for producing electricity typically use slender turbine blades extending radially from a hub. The hub is mounted to a shaft which drives a generator. Wind turbines are made larger and larger, and the static and dynamic loads on the turbine blades therefore increase.
In blade testing, static loads may be useful for evaluating stiffness and ultimate strength of a turbine blade. However, in practice, the load on a wind turbine blade varies constantly and to evaluate fatigue resistance of the blade, a cyclical load may be applied in large test facilities.
For economical reasons, it is desired to reduce the duration of a test. Typically, however, it is expensive and difficult to deflect the very large blades at large amplitudes and frequencies, and today several months are expected for each blade test.
During a typical test, a heavy load rotates eccentrically. The load is strapped to the blade at a location between the hub end and the tip end of the blade and therefore makes the blade oscillate. Unfortunately, the rotating mass increases the mass of the deflecting system, and the lowest resonance frequency is therefore reduced, and the duration of the test therefore increases further.